<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>三角形</title>
    <style>
        .box1 {
            width: 0px;
            height: 0px;
            border-top: 10px solid red;
            border-left: 10px solid blue;
            border-bottom: 10px solid green;
            border-right: 10px solid yellow;
        }

        .box2 {
            width: 0;
            height: 0;
            border: 10px solid transparent; /* transparent 是透明 */
            border-bottom-color: pink;
            margin: 10px auto;
        }
    </style>
</head>
<body>
    <div class="box1"></div>
    <div class="box2"></div>
</body>
</html>